Method of fabricating a semiconductor laser

ABSTRACT

A method of fabricating a semiconductor laser includes successively forming on a semiconductor substrate by crystal growth an active waveguide comprised of a compound semiconductor comprising a Group V element phosphorus, a thin-film layer comprised of a first-conductivity type compound semiconductor comprising a Group V element arsenic and a current blocking layer comprised of a second-conductivity type compound semiconductor comprising a Group V element arsenic. A mask is formed for selectively etching the current blocking layer in the form of a stripe. A buffer-etching step is formed on both the current blocking layer and the mask to expose a surface of the current blocking layer and the thin-film layer, the surface including a Group V element arsenic. An outer cladding layer comprising a first-conductivity type compound semiconductor having a Group V element arsenic is formed on the current blocking layer and the thin-film layer in an atmosphere having a Group V element arsenic. The method has characteristic features including carrying out the crystal growth only twice, minimizing the movement of impurities in crystals, regrowing the interface with a very little defect and forming a structure wherein the outer cladding layer has a smaller width at its portion near to the active waveguide.

This application is a division of application Ser. No. 07/756,016, filedSep. 6, 1991.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a semiconductor laser used in optical signalprocessing or optical communications, and a method of fabricating thesemiconductor laser.

2. Description of the Prior Art

FIG. 7 cross-sectionally illustrates the structure of a conventionalsemiconductor laser. Reference numeral 701 denotes a GaInP active layer,from which laser light of about 670 nm in wavelength is emitted.Reference numerals 702 and 703 each denote an AlGaInP cladding layer,having a function of confining the laser light to the GaInP active layer701 and guiding it to an emitting facet. A structure having such afunction is called a waveguide. These layers have a larger band gapwidth than the active layer, and hence at the same time have a functionof confining injected carriers in the active layer. Reference numeral704 denotes an n-type GaAs current blocking layer, which have twofunctions of preventing currents from passing through this layer andalso absorbing the laser light, and, as a result, have a function ofconfining the laser light to the area beneath a ridge 709 to let thelight guide therethrough. Namely, these form a waveguide structure whichguides the laser light in the lateral direction. Reference numeral 705denotes a p-type GaAs contact layer, which is in low-resistivity ohmiccontact with a p-type electrode 707. A semiconductor multi-layerstructure, comprised of these layers except the electrode 707, is formedon an n-type GaAs substrate 706 having the (100) plane on its surface.Reference numeral 708 denotes an n-type electrode, which is inlow-resistivity ohmic contact with the n-type GaAs substrate 706 (see,for example, KOGAKU (Optics), Vol. 19, pp.362-368, 1990).

Of these layers, the GaInP active layer 701 and the p-type and n-typeAlGaInP cladding layers 702 and 703 are formed by first crystal growth,the n-type GaAs current blocking layer 704 by second crystal growth, andthe p-type GaAs contact layer by third crystal growth.

In this device, the threshold current is 50 mA and the current needed togive a light output of 4 mW is 60 mA, at 25° C.

Fabricating the structure in which the laser light and current arelaterally confined in this way brings about a uniform gain, and hence itcan be expected that the wavefront of the laser light becomes flat andthe astigmatism thereof becomes small. The gain is meant to be thedegree to which the intensity of laser light is amplified when the laserlight is guided. The wavefront of the laser light swiftly advances atthe part where the gain is large. If the astigmatism is large, the spotformed when the light is focused can not be round, so that such laserlight can not be readily used as a light source for optical disks.

In such a conventional structure, however, the ridge is in the form of atrapezoid wherein the upside is short, and hence electric currents arelaterally spread while they flow from the upside to the active layer701. As a result, the gain in the active layer 701 becomes graduallysmaller toward the base end of the ridge 709. Hence, the wavefront ofthe laser light swiftly advances at the middle of the ridge, where theastigmatism is enlarged. Thus such laser light can not be readily usedas a light source for optical disks.

The n-type GaAs current blocking layer 704 absorbs the laser light andhence has a large guiding loss. The guiding loss is meant to be a lossthe laser light may undergo because of absorption or scattering duringits passing through the waveguide. This results in an increase inthreshold currents or operation currents.

The p-type AlGaInP cladding layer 702 is so high in both resistivity andthermal resistivity that it has been difficult to attain operation athigh temperatures or operation in a low droop. The droop is meant to bea gradual decrease in light intensity in one pulse amplitude that may becaused by a decrease in emittion efficiency due to heat generation, atthe time of pulsed operation.

The prior art structure also requires carrying out crystal growth threetimes. In the course of such crystal growth, the structure is heated toa high temperature of 600° C. to 700° C., so that impurities such aszinc present in crystals may move outside the layer because of theirdiffusion or the like. This may cause an increase in resistivity of thedevice or result in a poor efficiency for the confinement of carriers tothe active layer, tending to bring about a deterioration ofcharacteristics such as threshold currents, high-temperature operationand low-droop operation.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a device structure thatcan solve all the first, second, third and fourth problems as discussedabove and a device structure that can solve the first, third and fourthproblems.

Another object of the present invention is to provide a method offabricating a semiconductor laser, that can accomplish such devicestructures.

A first device structure according to the present invention comprises astructure comprising an active waveguide formed of a compoundsemiconductor comprising a Group V element phosphorus, comprised of anactive layer and two cladding layers that hold the active layer betweenthem, and a current confinement structure formed on the active waveguideby the use of a compound semiconductor comprising Group V elementarsenic; wherein the current confinement structure has at least astripelike conduction layer, a current blocking layer that holds bothsides of the stripelike conduction layer and a thin-film layer formedbeneath them in a layer thickness small enough not to affect thewaveguide performance of the active waveguide, the conduction layer hasat the same time a function as an outer cladding layer and has a widthmade narrower toward the active waveguide, and the current blockinglayer has a smaller refractive index than the conduction layer.

A second device structure according to the present invention comprises astructure comprising an active waveguide formed of a compoundsemiconductor comprising a Group V element phosphorus, comprised of anactive layer and two cladding layers that hold the active layer betweenthem, and a current confinement structure formed on the active waveguideby the use of a compound semiconductor comprising Group V elementarsenic; wherein the current confinement structure has at least astripelike conduction layer, a current blocking layer that holds bothsides of the stripelike conduction layer and a thin-film layer formedbeneath them in a layer thickness small enough not to affect thewaveguide performance of the active waveguide, the conduction layer hasat the same time a function as an outer cladding layer and has a widthmade narrower toward the active waveguide, and the current blockinglayer has the same band gap as, or a smaller band gap than, the photonenergy of emitted laser light.

A first fabricating method according to the present invention comprisessuccessively growing an active waveguide comprised of a compoundsemiconductor comprising a Group V element phosphorus, a thin-film layercomprised of a first-conductivity type compound semiconductor comprisingGroup V element arsenic and a current blocking layer comprised of asecond-conductivity type compound semiconductor, thereafter selectivelyetching only the current blocking layer in the form of a stripe using anetching mask formed thereon, subsequently buffer-etching only thecurrent blocking layer after removing the etching mask, to remove adamaged region on its surface, and then carrying out crystal growth toform thereon an outer cladding layer comprising Group V element arsenic.

A second fabricating method according to the present invention comprisessuccessively growing an active waveguide formed of a compoundsemiconductor comprising a Group V element phosphorus, a thin-film layerformed of a first-conductivity type compound semiconductor comprisingGroup V element arsenic, a current blocking layer formed of asecond-conductivity type compound semiconductor, a compoundsemiconductor protective layer and a compound semiconductor mask layer,thereafter selectively etching only the compound semiconductor masklayer in the form of a stripe using an etching mask formed thereon,subsequently etching only the compound semiconductor protective layerusing the compound semiconductor mask layer as a mask after removing theetching mask, thereafter simultaneously etching only the uncoveredportion of the current blocking layer and the compound semiconductormask layer, and then carrying out crystal growth to form thereon anouter cladding layer comprising Group V element arsenic.

According to the first device structure of the present invention, sincethe outer cladding layer is made narrower at its portion nearer to theactive waveguide, there can be no lateral spread of currents which hasoccurred in the prior art. Thus, the currents can be uniformly injectedinto the active layer. This can bring about a flat wavefront and a smallastigmatism. In addition, since the current blocking layer has a smallerrefractive index than the outer cladding layer, a lateral waveguidestructure with a very small waveguide loss can be readily fabricated andalso the threshold current or operation current can be decreased.Moreover, a material that is small in both resistivity and thermalresistivity can be used in the outer cladding layer, and hence it ispossible to prevent device temperature from rising.

The second device structure can obtain the effect that it is possible toattain a small astigmatism and prevent device temperature from rising,for the same reason as the above. As for the guiding loss, it can not bemade so much smaller than that in the first device structure, but thecontent of aluminum (Al) in the current blocking layer can be madesmaller than that in the first device structure. Hence, the seconddevice structure can also bring about the effect that the currentblocking layer can be less oxidized on its surface at the time of thesecond crystal growth and can be more readily grown.

According to the first fabricating method of the present invention, thecrystal growth may be carried out only twice, and hence the movement ofthe impurities in crystals does not easily occur. In addition, since thethin-film layer is not etched when the current blocking layer isbuffer-etched, the active waveguide layer can be protected from theetchant. Moreover, since the Group V element of the layer having beenuncovered when the second crystal growth is carried out is the same asthe Group V element of the outer cladding layer, a regrowth interface(herein an interface between layers formed by repeated crystal growth)with a very little defect can be readily obtained. Furthermore, sincethe sides of a groove in the current blocking layer can be readily madeoutwards sloping, uniform crystal growth can be readily carried out onthe surface in the second crystal growth and at the same time thestructure wherein the outer cladding layer has a smaller width at itsportion nearer to the active waveguide can be naturally formed.

The second fabricating method of the present invention can obtain, forthe same reason as the above, the effect that the movement of theimpurities in crystals does not easily occur, the active waveguide layercan be protected from the etchant, a regrowth interface with a verylittle defect can be readily obtained, uniform crystal growth can bereadily carried out on the surface in the second crystal growth, and thestructure wherein the outer cladding layer has a smaller width at itsportion nearer to the active waveguide can be naturally formed. Inaddition, since the compound semiconductor protective layer is notetched even when the compound semiconductor mask layer and the currentblocking layer are etched, the form of the current blocking layer can bedetermined accordingly. Since also the content of Al in the compoundsemiconductor protective layer can be made smaller than that in thecurrent blocking layer, the second fabricating method can also bringabout the effect that the current blocking layer can be less oxidized onits surface at the time of the second crystal growth and can be morereadily grown.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross section to illustrate the structure of a firstembodiment of the first device structure of the semiconductor laseraccording to the present invention.

FIG. 2 is a cross section to illustrate the structure of a secondembodiment of the first device structure of the semiconductor laseraccording to the present invention.

FIG. 3 is a cross section to illustrate the structure of a firstembodiment of the second device structure of the semiconductor laseraccording to the present invention.

FIG. 4 is a cross section to illustrate the structure of a secondembodiment of the second device structure of the semiconductor laseraccording to the present invention.

FIGS. 5A to 5D are cross sections to stepwise illustrate the firstmethod of fabricating the semiconductor laser according to the presentinvention.

FIGS. 6A to 6D are cross sections to stepwise illustrate the secondmethod of fabricating the semiconductor laser according to the presentinvention.

FIG. 7 is a cross section to illustrate the structure of a conventionalsemiconductor laser.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 cross-sectionally illustrates a first embodiment of the firstdevice structure of the semiconductor laser according to the presentinvention. Reference numeral 101 denotes an n-type GaAs substrate; and102, an n-type GaAs buffer layer of 0.5 micron thick. Reference numerals103, 104 and 105 denote an n-type AlGaInP cladding layer of 1 micronthick, a GaInP active layer of 0.06 micron thick and a p-type AlGaInPcladding layer of 0.3 micron thick, respectively. These three layersconstitute an active waveguide. Reference numeral 106 denotes a p-typeGaAs thin-film layer of 0.01 micron thick; 107, an n-type AlGaAs currentblocking layer of 0.7 micron thick; 108, a p-type AlGaInP outer claddinglayer of 1 micron thick; and 109, a GaAs contact layer of 3 micronsthick. Reference numerals 110 and 111 denote a p-type electrodecomprised of an alloy of Au and Zn and an n-type electrode comprised ofan alloy of Au and Ge, respectively, which can obtain a low ohmiccontact with respect to the p-type and n-type GaAs's, respectively.Sides 112 of a groove in the n-type AlGaAs current blocking layer areoutwards sloping, i.e., in the state that upward normals of the sides112 are always inclined in outward directions. A stripe width t at thebottom is about 3 microns.

The p-type AlGaInP outer cladding layer 108 (a conduction layer) mayhave composition represented by the formula Al_(x1) Ga_(1-x1) As(0≦x1≦1); the n-type AlGaAs current blocking layer 107, compositionrepresented by the formula Al_(y1) Ga_(1-y1) As (0≦y1≦1); and the p-typeGaAs thin-film layer 106, composition represented by the formula Al_(z1)Ga_(1-z1) As (0≦z1≦1).

The GaInP active layer 104 emits red laser light with a wavelength ofabout 670 nm. With respect to this wavelength, the GaInP active layer104 has a refractive index of about 3.6 and the AlGaInP cladding layers103 and 105 each have a refractive index of about 3.3. The p-typeAlGaInP outer cladding layer 108 is made to have substantially the samerefractive index as, or a smaller refractive index than, that of thep-type AlGaInP cladding layer 105. More specifically, the refractiveindex of AlGaAs is substantially 3.3 when composition x1 of Al in theouter cladding layer 108 is about 0.6, and gradually decreases with anincrease in x1. The refractive index of the current blocking layer 107is set smaller than that of the outer cladding layer 108. In otherwords, composition y1 of Al in the current blocking layer 107 is largerthan x1. When a difference in the refractive index is larger than about0.1, particularly good waveguide characteristics can be obtained. Inorder to obtain such a difference in refractive index, y1 must be madelarger than x1 by about 0.2. Accordingly, x1 should be in a value of 0.6to 0.8 (0.6≦x1≦0.8), and y1, a value of 0.8 to 1.0 (0.8≦x1≦1.0). Undersuch composition, the band gap is larger than the energy of laser light,and hence the laser light can not be absorbed. Operation and advantagesof this device will be described below.

To this semiconductor laser, a current is flowed from the p-typeelectrode 110 to the n-type electrode 111. As a result, holes flow fromthe GaAs contact layer 109 to the p-type AlGaInP outer cladding layer108. However, the pn junction between the n-type AlGaAs current blockinglayer 107 and the p-type GaAs thin-film layer 106 is brought into areverse bias, and hence no current flows here. The current insteadpasses through the part held between the two outwards sloping sides 112,so that the current can be gradually narrowed down. This part has aresistivity and a thermal resistivity that are sufficiently lower thanthe AlGaInP layers, and hence the heat can be less generated and anyheat once generated can be quickly dissipated. Since the p-type AlGaInPcladding layer 105 has a small thickness, the current injected here islittle spread and immediately injected into the GaInP active layer 104.Thus, the wavefront of the laser light can be flat. The laser light isalso spreadingly guided through the p-type AlGaInP outer cladding layer108 and the n-type AlGaAs current blocking layer 107. Since, aspreviously stated, the former has a higher refractive index than thelatter, an average refractive index the laser light may feel in thelateral direction becomes higher at the part where the former ispresent, and the laser light is guided mainly in confinement to thispart and the active waveguide positioned right beneath it. The laserlight may also be spread to the p-type GaAs thin-film layer 106. TheGaAs thin-film layer 106 has a high refractive index and also absorbsthis laser light, but has a thickness made as small as 0.01 micron.Hence it little affects the guiding of the laser light. Therefore theguiding loss due to the absorption little occurs in this structure, sothat it become possible to decrease the threshold currents or operationcurrents.

FIG. 2 cross-sectionally illustrates a second embodiment of the firstdevice structure of the semiconductor laser according to the presentinvention. The layers denoted by reference numerals common to those inFIG. 1 are composed of the same materials. What is different is that thestructure has an n-type GaAs protective layer 201. This n-type GaAsprotective layer may have composition represented by Al_(z1) Ga_(1-z1)As (0≦z1≦1). Although the presence of this layer results in an increasein the number of layers, the form of the outwards sloping sides 112 canbe more delicately controlled because of the advantages on thefabrication as stated later. As a result, it is readily possible to morenarrow a width t' than the width t in FIG. 1. It therefore becomespossible to more decrease the threshold currents or operation currents.Other performances and advantages are the same as those in the firstembodiment.

FIG. 3 cross-sectionally illustrates a first embodiment of the seconddevice structure of the semiconductor laser according to the presentinvention. Reference numeral 301 denotes an n-type GaAs substrate; and302, an n-type GaAs buffer layer of 0.5 micron thick. Reference numerals303, 304 and 305 denote an n-type AlGaInP cladding layer of 1 micronthick, a GaInP active layer of 0.06 micron thick and a p-type AlGaInPcladding layer of 0.3 micron thick, respectively. These three layersconstitute an active waveguide. Reference numeral 306 denotes a p-typeGaAs thin-film layer of 0.01 micron thick; 307, an n-type AlGaAs currentblocking layer of 0.7 micron thick; 308, a p-type AlGaInP outer claddinglayer of 1 micron thick; and 309, a GaAs contact layer of 3 micronsthick. Reference numerals 310 and 311 denote a p-type type electrodecomprised of an alloy of Au and Zn and an n-type electrode comprised ofan alloy of Au and Ge, respectively. Sides 312 of a groove in the n-typeAlGaAs current blocking layer are outwards sloping. A stripe width t atthe bottom is about 4 microns.

The p-type AlGaInP outer cladding layer 308 (a conduction layer) mayhave composition represented by the formula Al_(x2) Ga_(1-x2) As(0≦x2≦1); the n-type AlGaAs current blocking layer 307, compositionrepresented by the formula Al_(y2) Ga_(1-y2) As (0≦y2≦1); and the p-typeGaAs thin-film layer 306, composition represented by the formula Al_(z2)Ga_(1-z2) As (0≦z2≦1).

The GaInP active layer 304 emits red laser light with a wavelength ofabout 670 nm. With respect to this wavelength, the GaInP active layer304 has a refractive index of about 3.6 and the AlGaInP cladding layers303 and 305 each have a refractive index of about 3.3. The p-typeAlGaInP outer cladding layer 308 is made to have substantially the samerefractive index as, or a smaller refractive index than, that of thep-type AlGaInP cladding layer 305. More specifically, composition x2 ofAl in the outer cladding layer 308 is made to be not less than 0.6.Under such composition, the band gap is larger than the energy of laserlight, and hence the laser light can not be absorbed. More specifically,this occurs when composition y2 of Al in the current blocking layer 307is not more than about 0.3. When this layer has such a low Alcomposition, the surface of this layer is not easily oxidized, so thatit becomes very easy to carry out the second crystal growth. Operationand advantages of this device will be described below.

To this semiconductor laser, a current is flowed from the p-typeelectrode 310 to the n-type electrode 311. As a result, holes flow fromthe GaAs contact layer 309 to the p-type AlGaInP outer cladding layer308. However, the pn junction between the n-type AlGaAs current blockinglayer 307 and the p-type GaAs thin-film layer 306 is brought into areverse bias, and hence no current flows here. The current insteadpasses through the part held between the two outwards sloping sides 312,so that the current can be gradually narrowed down. This part has aresistivity and a thermal resistivity that are sufficiently lower thanthe AlGaInP layers, and hence the heat can be less generated and anyheat once generated can be quickly dissipated. Since the p-type AlGaInPcladding layer 305 has a small thickness, the current injected here islittle spread and immediately injected into the GaInP active layer 304.Thus, the wavefront of the laser light can be flat. The laser light isalso spreadingly guided through the p-type AlGaInP outer cladding layer308 and the n-type AlGaAs current blocking layer 307. Since, aspreviously stated, the n-type AlGaAs current blocking layer absorbs thelaser light, the laser light is guided mainly through p-type AlGaInPouter cladding layer 308 and the active waveguide positioned rightbeneath it. The laser light may also be spread to the p-type GaAsthin-film layer 306. The GaAs thin-film layer 306 has a high refractiveindex and also absorbs this laser light, but has a thickness made assmall as 0.01 micron. Hence it little affects the guiding of the laserlight. In this structure, the width t through which the current isinjected can be made smaller than that in the prior art, and hence itbecome possible to decrease the threshold currents or operationcurrents.

FIG. 4 cross-sectionally illustrates a second embodiment of the seconddevice structure of the semiconductor laser according to the presentinvention. The layers denoted by reference numerals common to those inFIG. 1 are composed of the same materials. What is different is that thestructure has an n-type GaAs protective layer 401. This n-type GaAsprotective layer may have composition represented by Al_(z2) Ga_(1-z2)As (0≦z2≦1). Although the presence of this layer results in an increasein the number of layers, the form of the outwards sloping sides 312 canbe more delicately controlled because of the advantages on thefabrication as stated later. As a result, it is readily possible to morenarrow a width t' than the width t in FIG. 3. It therefore becomespossible to more decrease the threshold currents or operation currents.Other performances and advantages are the same as those in the firstembodiment.

FIGS. 5A to 5D are cross sections to stepwise illustrate the firstmethod of fabricating the semiconductor laser according to the presentinvention. As an example, the procedure of preparing the device shown inFIG. 1 is shown there. The layers denoted by reference numerals commonto those in FIG. 1 are composed of the same materials. FIG. 5A shows astate in which first crystal growth has been completed. The crystalgrowth is carried out by metal-organic vapor phase epitaxy. Next, aphotoresist 501 is coated on the surface. The photoresist is selectivelyremoved by photolithography in the form of a stripe with a width of 3microns. Using this photoresist as an etching mask, the n-type AlGaAscurrent blocking layer 107 is selectively etched with hydrofluoric acid.As a result, a groove with outwards sloping sides 112 is formed. Thehydrofluoric acid has so low an etching speed on GaAs or AlGaAs with asmall Al content (z1) that it can be used as an etchant for selectivelyetching the AlGaAs with a large Al content. In particular, the selectiveetching becomes very easy when the Al content z1 is smaller than 0.3.Hence, the p-type GaAs thin-film layer 106 is not etched withhydrofluoric acid. This state is shown in FIG. 5B. Next, after thephotoresist has been removed using acetone, the surface of the n-typeAlGaAs current blocking layer 107 is etched with hydrofluoric acid for ashort time. This etching is called buffer etching, which is carried outfor the purpose of preventing good second crystal growth from beingobstructed by surface defects or stains produced as a result of exposureto the air or formation of the photoresist layer. This buffer etchingmakes the width of a groove a little larger to give the width of about 3microns. At this stage also, the p-type GaAs thin-film layer 106 is notetched with the hydrofluoric acid. This state is shown in FIG. 5C. Next,the second crystal growth is carried out thereon. Since the planes onwhich the crystal growth is effected are comprised of AlGaAs and GaAs inwhich Group V element is only As, arsine may only be introduced in orderto prevent the Group V element from escaping from the layers to thegaseous phase in the course of temperature rise. The arsine issuccessively continued flowing also when the p-type AlGaInP outercladding layer 108 is grown, so that a good regrowth interface 502 canbe obtained without any interruption of the starting material gas flow.In addition, since the terraces on the surface have outwards slopingsides 112 only, uniform crystal growth can be effected because ofpresence of no surfaces or corners that may cause a difficulty incarrying out the crystal growth, so that the p-type AlGaInP outercladding layer 108 and the p-type contact layer 109 can be readilyformed. Thereafter, the p-type electrode 110 is formed by vacuumdeposition. After the back surface of the n-type GaAs substrate 101 hasbeen abraded to give a thickness of about 100 microns, the n-typeelectrode 111 is further formed by vacuum deposition, followed byconvertion of these electrodes into alloys at about 400° C. to completethe process. This state is shown in FIG. 5D.

Characteristic features of the above first method of fabricating thesemiconductor laser of the present invention can be summarize asfollows: The crystal growth may be carried out only twice, and hence themovement of the impurities in crystals does not easily occur; thethin-film layer can protect the active waveguide layer from the etchant;a regrowth interface with a very little defect can be readily obtained;since the sides of the groove in the current blocking layer can bereadily made outwards sloping, uniform crystal growth can be readilycarried out on the surface in the second crystal growth; and thestructure wherein the outer cladding layer has a smaller width at itsportion nearer to the active waveguide can be naturally formed. Needlessto say, the first method of fabricating the semiconductor laser of thepresent invention as described above can be also applied to the casewhen the device shown in FIG. 3 is fabricated.

FIGS. 6A to 6D are cross sections to stepwise illustrate the secondmethod of fabricating the semiconductor laser according to the presentinvention. As an example, the procedure of preparing the device shown inFIG. 2 is shown there. The layers denoted by reference numerals commonto those in FIG. 2 are composed of the same materials. FIG. 6A shows astate in which first crystal growth has been completed. The crystalgrowth is carried out by metal-organic vapor phase epitaxy. Referencenumeral 601 denotes a mask layer comprised of AlGaAs with an Al contentrelatively as high as 0.7. Next, a photoresist 602 is coated on thesurface. The photoresist is selectively removed by photolithography inthe form of a stripe with a width of 3 microns. Using this photoresistas an etching mask, the mask layer 601 is selectively etched withhydrofluoric acid. At this time, the n-type GaAs protective layer 201 isnot etched with hydrofluoric acid. This state is shown in FIG. 6B. Next,after the photoresist 602 has been removed using acetone, the uncoveredpart of the n-type GaAs protective layer 201 is etched using the masklayer 601 as an etching mask. As an etchant, a selective etchant is usedwhich is capable of selectively etching GaAs or AlGaAs with a small Alcontent, as exemplified by a solution comprising a mixture of liquidammonia and oxygenated water. Next, the n-type AlGaAs current blockinglayer 107 and the mask layer 601 are simultaneously etched withhydrofluoric acid. As a result, a groove with outwards sloping sides 112is formed in the n-type AlGaAs current blocking layer 107. Since the Alcontent in the mask layer 601 is sufficiently higher than that in then-type GaAs protective layer 201, the mask layer 601 can be completelyremoved without etching of the n-type GaAs protective layer 201. Thisremoval of the mask layer 601 can bring about removal of surface defectsor stains produced as a result of exposure to the air or formation ofthe photoresist layer, and hence the second crystal growth can becarried out in a good state. This selective etching does not broaden atall the width of the groove, and hence a groove with a smaller widththan that in the first fabricating method can be formed. In the case ofthis example, the groove is formed in a width of about 2 microns. Atthis stage also, the p-type GaAs thin-film layer 106 is not etched withthe hydrofluoric acid. This state is shown in FIG. 6C. Next, the secondcrystal growth is carried out thereon. Since the planes on which thecrystal growth is effected are comprised of AlGaAs and GaAs in whichGroup V element is only As, arsine may only be introduced in order toprevent the Group V element from escaping from the layers to the gaseousphase in the course of temperature rise. The arsine is successivelycontinued flowing also when the p-type AlGaInP outer cladding layer 108is grown, so that a good regrowth interface 502 can be obtained withoutany interruption of the starting material gas flow. In addition, sincethe terraces on the surface have outwards sloping sides 112 only,uniform crystal growth can be effected because of presence of nosurfaces or corners that may cause a difficulty in carrying out thecrystal growth, so that the p-type AlGaInP outer cladding layer 108 andthe p-type contact layer 109 can be readily formed. Moreover, sincealmost the whole surface is covered with the n-type GaAs protectivelayer 201 that can not be easily oxidized, good crystal growth can beeffected without taking any particular care to oxidation. Thereafter,the p-type electrode 110 is formed by vacuum deposition. After the backsurface of the n-type GaAs substrate 101 has been abraded to give athickness of about 100 microns, the n-type electrode 111 is furtherformed by vacuum deposition, followed by conversion of these electrodesinto alloys at about 400° C. to complete the process. This state isshown in FIG. 6D.

Characteristic features of the above second method of fabricating thesemiconductor laser of the present invention can be summarize asfollows: The semiconductor laser protective layer makes the surfaceoxidation not easily occur in the course of crystal growth and alsomakes it possible to form a narrow groove in the current blocking layerin a good controllability; the movement of the impurities in crystalsdoes not easily occur; the active waveguide layer can be protected fromthe etchant; a regrowth interface with a very little defect can bereadily obtained; uniform crystal growth can be readily carried out onthe surface in the second crystal growth; and the structure wherein theouter cladding layer has a smaller width at its portion nearer to theactive waveguide can be naturally formed. Needless to say, the secondmethod of fabricating the semiconductor laser of the present inventionas described above can be also applied to the case when the device shownin FIG. 4 is fabricated.

Stated additionally, in the devices or methods as shown in FIGS. 1 to 6,there is no problem if the active layer is made to comprise an AlGaInPlayer, a laminated structure comprised of thin films of AlGaInp andGaInp, or a laminated structure comprised of two or more kinds ofAlGaInP thin films. There is also no problem if the n-type and p-typeAlGaInP cladding layers have composition different from each other. Itis also possible to carry out the same selective etching without anyproblem even when AlGaAs with a low Al content is used in the thin-filmlayer. The crystal growth also is not necessarily be limited to themetal-organic vapor phase epitaxy, and there is no problem if othercrystal growth process such as molecular beam epitaxy is used. In thefirst device structure, the current blocking layer is not necessarilycomprised of AlGaAs, where the same effect can be obtained and also thesame fabrication method can be applied if it is comprised of AlAs. Theprotective layer is also not necessarily be of n-type, and may be ofp-type without any problem.

According to the present invention, it is possible to achieve a smallastigmatism, a low threshold current and a low operation current, sothat it is possible to readily fabricate a semiconductor laser havingalso the features that it can attain operation at high temperatures oroperation in a low droop.

It is also possible to readily fabricate a semiconductor laser havingthe characteristic features that the crystal growth may be carried outonly twice, the movement of the impurities in crystals does not easilyoccur, a regrowth interface with a very little defect can be readilyobtained, and the structure wherein the outer cladding layer has asmaller width at its portion nearer to the active waveguide can benaturally formed.

What is claimed is:
 1. A method of fabricating a semiconductor laser bya process comprising the steps of:1) successively forming on asemiconductor substrate by crystal growth an active waveguide comprisedof a compound semiconductor comprising a Group V element phosphorus, athin-film layer comprised of a first-conductivity type compoundsemiconductor comprising a Group V element arsenic and a currentblocking layer comprised of a second-conductivity type compoundsemiconductor comprising a Group V element arsenic; 2) forming a maskfor etching the current blocking layer on the thin-film layer; 3)selectively etching said current blocking layer in the form of a stripe;4) buffer-etching said current blocking layer and said masksimultaneously by the use of an etchant capable of selectively etchingthe current blocking layer and said mask and exposing a surface of saidcurrent blocking layer and said thin-film layer wherein a Group Velement consisting of arsenic appears at said surface; and 5) forming onsaid current blocking layer and said thin-film layer by crystal growthan outer cladding layer comprised of a first-conductivity type compoundsemiconductor comprising a Group V element arsenic in an atmospherehaving a Group V element consisting of arsenic.
 2. A method offabricating a semiconductor laser according to claim 1, wherein saidcrystal growth is carried out by metal-organic vapor phase epitaxy.
 3. Amethod of fabricating a semiconductor laser by process comprising thesteps of;successively forming on a semiconductor substrate by crystalgrowth an active waveguide comprised of a compound semiconductorcomprising a Group V element phosphorus, a thin-film layer comprised ofa first-conductivity type compound semiconductor comprising a Group Velement arsenic, a current blocking layer comprised of asecond-conductivity type compound semiconductor, a compoundsemiconductor protective layer and a compound semiconductor mask layer;selectively etching said compound semiconductor mask layer in the formof a stripe; selectively etching the uncovered part of said compoundsemiconductor protective layer; simultaneously selectively etching theuncovered part of said current blocking layer and said compoundsemiconductor mask layer and exposing a surface of said current blockinglayer, said protective layer and said thin-film layer wherein said aGroup V element insisting of arsenic appears at said surface; andforming on the sides of a groove in said current blocking layer, saidprotective layer and said thin-film layer by crystal growth an outercladding layer comprised of a first-conductivity type compoundsemiconductor comprising a Group V element arsenic in an atmospherehaving a Group V element consisting of arsenic.
 4. A method offabricating a semiconductor laser according to claim 3, wherein saidcrystal growth is carried out by metal-organic vapor phase epitaxy.